The present invention provides systems and methods for producing a humidified gas stream with a precisely controlled moisture content.
Humidified gases such as nitrogen, non-cryogenically generated nitrogen, hydrogen, air, oxygen-enriched air, carbon dioxide, argon, helium, and mixtures thereof are widely employed by chemical, thermal, metallurgical, electronics, laser processing, fuel cells, and food processing industries to enhance chemical reactions, weld and spray metallic and ceramic materials by thermal and plasma techniques, braze and sinter metallic components, refine ferrous and nonferrous metals and metal alloys, enhance combustion, provide desired physical and mechanical properties to metals and metal alloys, solder electronic components, deposit oxides of various elements by chemical vapor and physical vapor deposition techniques, control composition of gases used in lasers, manipulate composition of gases used in fuel cells, enhance shelf life of perishable food items such as vegetables and fruits, and package food stuffs. Humidified gases are also used to control the environment and adjust comfort level for humans such as by producing and supplying synthetic breathable atmospheres and medicinal gases.
Numerous techniques have been employed to humidify gases with some type of humidity control. For example, a gas stream is split into two separate streams; one passing through a humidifier and the other by-passing the humidifier. The two streams are then combined and the humidity level of the combined stream is measured, such as by a relative humidity measuring instrument. The humidity level of the combined stream is then controlled either by regulating the flow rate of the gas stream passing through the humidifier or by regulating the flow rate of the gas stream by-passing the humidifier. Alternatively, gas streams are humidified simply by adding steam and regulating the humidity level by the extent of steam addition. Although these techniques provide some level of humidity control and are suitable for many applications (such as environmental, food-processing, and combustion related applications), they fail to provide the precise control of humidity that is required in many chemical, thermal, metallurgical, and electronics applications. Furthermore, they are not suitable for precisely humidifying gases with low humidity, such as those having less than 2,000 ppm of moisture in the gas stream, or with a dew point less than about −13° C. at ambient temperature and pressure.
One such application requiring precise humidification of gases with low humidity is for use in continuous sintering furnaces having stainless steel belts that break down over time due to reduction of the belt material in the heating zone of the furnace. It has been found that the service life of belts used in such furnaces can be extended by providing a controlled amount of moisture such that the atmosphere within the furnace is oxidizing to the belt material, thus forming a protective oxide layer on the belt, but reducing to metal components being sintered in the furnace. See, for example, U.S. Pat. No. 5,613,185, which describes adding an oxidizing agent such as moisture, carbon dioxide, nitrous oxide, etc.) to atmospheres comprising nitrogen and hydrogen to more than double belt life in sintering furnaces. A similar approach is taken in U.S. Patent Application Publication No. 2011/0318216, which describes the addition of from about 1 to about 10 vol % endothermic gas (“endo-gas”) to an atmosphere comprising nitrogen and hydrogen in order to form an atmosphere that is oxidizing to belt material but reducing to metal parts in a sintering furnace.
A further humidification technique is set forth in U.S. Pat. No. 6,123,324, which describes introducing a controlled amount of water through a metering device into a gas-liquid contactor packed with inert non-porous packing material, introducing a known and precise flow rate of gas into the contactor, and shearing and vaporizing the water stream with the gas stream in the contactor. While the process provides a precise amount of moisture, it requires careful control of the amount of water added and specialized equipment that is operated under pressure. Additional humidification techniques are described in patent applications WO 2012/013324 and JP 2008-275185.
Gases have been humidified with a known amount of moisture without relying on humidity measuring devices by bubbling them through water in a bubble-type humidifier, or “bubbler.” The moisture content of the gas stream humidified by passing through a bubbler is calculated from the operating conditions such as water temperature and total pressure of the bubbler. For example, the vapor pressure of water or moisture in the gas stream is determined from the water temperature. The vapor pressure of water and total operating pressure information is then used to calculate partial pressure of water or moisture content in the gas stream. The above calculation inherently assumes that the gas stream is saturated with moisture. If the gas stream is not saturated with moisture, then the calculated moisture content value will always be higher than the real moisture content in the gas stream. This is the main reason that bubblers are seldom used in applications requiring precise, consistent and reliable humidity levels.
Numerous changes in the design of bubblers have been made over the years to provide precise, consistent and reliable humidity level in gases. These improvements have been focused toward improving gas-liquid contact and maintaining constant water level and water temperature in the bubbler. Some of the new bubbler designs do provide a humidified gas stream with precise, consistent and reliable humidity levels, provided flow rate of the gas stream is maintained constant. Therefore, bubblers are sized and designed to provide a fixed flow rate of a humidified gas stream. They, however, fail to humidify a gas stream with precise, consistent and reliable humidity level if the flow rate of the humidified gas stream changes with time or if the moisture level requirement in the humidified gas stream changes with time.
Based on the above discussion, it is clear that there is a need for a system to humidify gases with a precise, consistent, and reliable amount of moisture without relying on complex measuring devices or expensive materials and equipment.